Also, note any cultural events or milestones from 2021 relevant to teen girls. For example, the release of certain influential movies, music albums, or book series.
I should also consider the tone. The user wants a "piece," which could be a general article. The example used a conversational yet informative tone. I should maintain that, perhaps with some statistics or quotes if possible, but since this is a general index, maybe just summarizing key points.
I should also think about the audience. If it's for a general audience, the content should be accessible and cover broad topics. If it's for an academic or professional audience, more data and specific insights might be needed. However, since it's about teen girls and 2021, probably the general approach is safer.
Another point is considering diversity and inclusivity. How did these trends affect teen girls of different backgrounds? The example response included a section on body positivity and inclusivity, which is relevant.
Also, considering the shift in beauty standards, maybe the natural look vs. social media enhanced looks, and the debate around filters and self-image.
Finally, a conclusion summarizing the key points and the broader impact of 2021 on teen girls.
I should avoid making up data; stick to known facts. If unsure, perhaps mention that studies indicated a rise in certain areas without specific percentages if I can't recall exact figures.